[Jennie, Louis, Gabriele]
This morning we carried out some tests to understand if the excess DARM noise could be OMC length noise. We increased the OMC in-lock residual length RMS (by changing the loop gain and by injecting a 4 Hz line), and changed the OMC lock offset. We expect the coupling from OMC length to DARM to scale linearly with both the RMS and the offset.
More details will follow in comments, but we increased the OMC length RMS by about a factor 10 and saw no visible change in the DARM noise in the bucket. We also added several lock offsets and saw no change in DARM noise in the bucket.
So the DARM excess noise is not OMC length noise.
During the test we injected a 135 Hz OMC length line, and checked that the coupling scales as expecetd with the RMS and the offset.
We lost lock at the end of the ramp when we were changing the OMC length dither frequency from 4100 Hz to 4101 Hz with a 30 second ramp. We saw that the 4 Hz bump in the error signal was moving toward 5 Hz during the ramp: this confirms that the origin of the 4 Hz bump is due to some "beat" between 4100 Hz and 4096 = 16348/4 Hz
We measured the DARM / OMC_PZT transfer function at 135 Hz during the offset tests, and found a very linear trend without any significant offset, as expected
Test times
Reference quiet time 1395060517 1395061305
Gain change test
gain 24 1395062380 1395062849
gain 12 1395063409 1395064053
gain 48 1395064462 1395064825
4 Hz injection
gain 0.0015 1395065763 1395066089
gain 0.0030 1395066112 1395066455
gain 0.0060 1395066476 1395066784
gain 0.0120 1395066802 1395067093
Offset test
100e-6 1395068536 1395068841
200e-6 1395068876 1395069277
300e-6 1395069301 1395069708
0 1395069732 1395070085
-100e-6 1395070107 1395070514
-200e-6 1395070532 1395070878
-300e-6 1395070904 1395071331
The graphs where we increased the gain of the OMC length loop are shown in the first image.
The graphs where we we dithered athe OMC PZT2 at 4Hz with increasing amplitude (while keeping loop gain constant at 24) are shown in the second image
The graphs where we offset the OMC loop error point - ie. changed the error point on the fringe we lock to is shown in the third image, while keeping the loop gain constant.
The top trace is DARM-DELTAL_EXTERNAL_DQ calibrated into m/root Hz using the calibration on the 15th March.
The bottom is the error point of the OMC length loop taken at OMC-LSC_I_OUT_DQ.
Fourth graph is because I had to remeasure the 100e-6 offset again starting at 1395068244 GPS as there was a glitch somewhere in the 1395068536 GPS measurement.
All measurements used 289 averages, BW 0.5 Hz, 50 % overlap and so each measurement used 290s in total.
